feat(training-agent): wire BuyerAgentRegistry on all v6 tenants + sandbox-mode adaptation for SDK 6.8.0

[bokelley]

Summary

Phase 1 BuyerAgentRegistry adoption (per adcp-client#1269). Unblocked by SDK 6.8.0's extra-forwarding fix — issue #1484 (filed earlier in this session) → PR #1488 → shipped in 6.8.0.

Also bundles the SDK 6.8.0 sandbox-mode adaptation: 6.8.0 added a new comply_test_controller gate (isSandboxOrMockAccount) that requires resolved accounts to be in sandbox/mock mode. Training-agent operates as a public sandbox by design but didn't communicate that on the wire. Without this fix, the SDK bump alone would FORBIDDEN-reject every comply-controller storyboard. Pre-push storyboard floors confirm: all six tenants meet baselines after the fix.

What landed

1. SDK pin bumped to ^6.8.0.
2. Bearer authenticator stamps extra: { demo_token: token } on the AuthPrincipal returned for demo-* test bearers (server/src/training-agent/index.ts). The token doesn't survive AdcpCredential normalization (api_key.key_id is SHA-256 hashed); extra is the documented escape hatch for prefix-based test conventions.
3. New buyer-agent-registry.ts — BuyerAgentRegistry.bearerOnly resolver that reads extra.demo_token, delegates to commercial-relationships.ts for the principal → relationship lookup, and returns a BuyerAgent record with the appropriate billing_capabilities Set.
4. All six v6 tenant platforms wire agentRegistry: trainingBuyerAgentRegistry. The framework calls agentRegistry.resolve(authInfo) once per request before accounts.resolve and threads the resolved record through ctx.agent to platform handlers — forward-compat for SDK Phase 2 framework-level enforcement.
5. sandbox: true added to all accounts.resolve return values across the six v6 platforms (both ref == null synthetic-public-sandbox and brand/operator-resolved paths). Required by SDK 6.8.0's new comply_test_controller gate.

No gate-logic changes. commercial-relationships.ts remains the source of truth; legacy /mcp keeps consuming it via handleSyncAccounts. The v6 path now also has ctx.agent.billing_capabilities populated.

Tests

• 6 new unit tests in buyer-agent-registry.test.ts covering all branches (passthrough/agent-billable prefix resolution, unrecognized prefix → null, missing extra → null, defensive type-checks, http_sig refusal).
• Existing sync-accounts-gates.test.ts (11 tests) and account-handlers.test.ts (22 tests) all pass — proves gate logic still fires correctly.
• Pre-push storyboard floors: all six tenants meet baselines (signals: 65 clean / 70 steps; sales: 65/74; governance: 64/70; creative: 65/83; creative-builder: 59/65; brand: 65/14).
• Targeted suite: 39/39 pass.
• Typecheck clean.

Phase 2 outlook

When SDK Phase 2 ships framework-level enforcement, the only remaining work is to delete the gate logic from account-handlers.ts and let the framework reject BILLING_NOT_PERMITTED_FOR_AGENT against ctx.agent.billing_capabilities directly. commercial-relationships.ts stays as the registry's data source.

Cross-links

• adcp-client#1484 — issue filed for the bearer-path gap (closed)
• adcp-client#1488 — SDK fix (merged, in 6.8.0)
• adcp#3984 — v6 accounts.upsert wiring (merged, prerequisite for this work)
• adcp#3851 — original gate implementation (merged)

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